Al Jazeera's Aela Callan, reporting from Bangkok, said: "About 200 protesters went into the the building staging a sit-in ... Protesters that have been laying siege to Bangkok's tourist area for the past few days."
Callen said the protesters chose a sensitive area for their activity, where many government and sovereign bodies are located, including the headquarters of the Thai Royal Army.
Despite warnings they could face a year in jail, thousands of other protester remained encamped in an area of upscale department stores and luxury hotels.
"We won't leave. We have sent our lawyer to the court and will submit an objection immediately if the court issues any order to force us to leave," Nattawut Saikua, a protest leader, said.
The government filed a court order on Monday to disperse the crowd and planned to file another to arrest leaders of the rally, which it says violates Thailand's Internal Security Act, imposed last month to maintain order during the protests.
However, Jatuporn Prompan, another protest leader, said demonstrators would maintain bases within both Bangkok's commercial heart and the separate historic quarter of the city - where they began a protest on March 12.
He also said that big businesses would be "in big trouble" if they did not sever their connections to the government.
There has been increasingly criticism of the impact that the long-running protests are having on the capital's economy, with retailers forced to shut their doors and roads blocked.
Thai stocks, which have climbed 81 per cent over the past 12 months, were about 0.6 per cent weaker on concerns over the prolonged protest, bucking gains in most other Asian markets. The baht currency eased about 0.2 per cent to 32.38 per dollar in thin trade.
Chakkrit Charoenmetachai, an analyst with Globlex Securities said: "The impact on retail, hotel and tourism-related sectors seems unavoidable and we should see selling pressure in these sectors."
"The impact on retail, hotel and tourism-related sectors seems unavoidable"
Tourism supports about five percent of the economy. Retail and hotel firms fell to three-week lows.
Minor International, Thailand's largest hotelier, lost 0.9 per cent, hotelier the Erawan Group which runs a Grand Hyatt franchise, fell 2.5 per cent and Central Plaza Hotel dropped 1.9 per cent. All four have properties near the protest sites.
The red shirts have demanded that Abhisit dissolve the government and call earlt elections, but the prime minister has refused, saying that holding a peaceful poll now would be difficult, given the tensions.
He did, however, repeat his offer to dissolve parliament in December, a year early.
Analysts say Abhisit would probably lose an election if it were held now, raising investment risks in Southeast Asia's second-biggest economy following a $1.6 billion surge of foreign investment in Thai stocks over the past five weeks.
The "red shirts" say Abhisit has no popular mandate and came to power illegitimately, heading a coalition with tacit military support after courts dissolved a party which supported Thaksin Shinawatra, the ousted former prime minister.
Abhisit says he was voted into office by the same parliament that picked his Thaksin-allied predecessors.